The invention is concerned, more especially, with pistons suitable for use in dispensing systems that also function to store a substance to be dispensed until it is required for use. Dispensing systems of that type are known, for example, in the field of dentistry where dental materials such as impression materials, restoration materials and filler materials are often supplied in cartridges comprising a storage chamber in which the material, or a component thereof, is contained. Such a cartridge may be provided, at its front end, with a sealed dispensing outlet and, at its rear end, with a piston that closes the storage chamber. When the dental material is required, the dispensing outlet is opened and the piston is driven into the storage chamber to dispense the contents through the outlet. The dental material may consist of one component only, or of two or more components that are to be mixed together for use: in the latter case, the components may be contained in separate storage chambers within the cartridge. Increasingly, in the dental field, single-use cartridges are employed with hand-held applicators that enable a dental practitioner to dispense a dental material from the cartridge directly to a selected site within a patient's mouth.
Dispensing cartridges suitable for dental materials are described, for example, in WO 2005/016783 and WO 2007/047381.
A problem that is frequently encountered when filling a substance into a dispensing system in which it is intended to be stored is that of removing air from the storage chamber and ensuring that no voids or air bubbles are trapped in the stored substance. The presence of air may have an adverse effect on the shelf life of the stored substance, and the presence of voids or air bubbles may have an adverse effect on the accuracy with which metered quantities of the substance can be dispensed. One conventional way of addressing this problem is to assemble the dispensing system under vacuum but this can result in the evaporation of ingredients from the substance to be stored and a consequent alteration in its composition. Another way of addressing the problem is to fill the system through its dispensing outlet but this method can only be used with systems that are appropriately configured.
It has also been proposed to address the problem by using a so-called vented piston in the storage chamber of the dispensing system. Storage and dispensing systems that incorporate some form of vented piston are described, for example, in U.S. Pat. No. 4,632,672; U.S. Pat. No. 5,178,305 and U.S. Pat. No. 6,899,254 and in WO 01/94028, as well as the above-mentioned WO 2005/016783 and WO 2007/047381.